There has been known an image forming apparatus such as a copying machine or a multifunction peripheral that has a function of (i) detecting interleaving papers from a plurality of bundles of documents, each of which bundles is made up of at least one document and which bundles are stacked so that an interleaving paper is placed between any two adjacent ones of the plurality of bundles and (ii) carries out an output process with respect to each of the bundles of documents, that is, a separating function. For example, Patent Literature 1 discloses the following technique. In a case where (i) an interleaving paper is detected in a double-side copying mode and (ii) the number of documents which is counted just before a detected interleaving paper is odd, recording papers, in a duplex tray, whose surfaces have already been printed are discharged with no print on the other of the surfaces, and then each of documents, placed under the interleaving paper, are subjected to a process to be carried out in the double-side copying mode. Note that, in a case an interleaving paper detecting mode is selected, it is determined whether or not each document is blank, and an interleaving paper is detected when a document has been determined to be blank.
A technique disclosed in Patent Literature 1 makes it possible to carry out double-side copy for individual groups of documents without copying, on both surfaces of a single recording paper, the last page of a group of odd-numbered single-side printed documents (a bundle of documents) and the first page of a subsequent group of documents. That is, according to the technique disclosed in Patent Literature 1, in a case where an interleaving paper is detected, a recording paper whose single side has been copied and whose other side has not been copied is discharged (outputted), and therefore the first page of the group of documents following the interleaving paper is inevitably copied on a surface of another recording paper.